1. Field of the Invention
The invention relates generally to improvements in motor vehicle chassis construction. More particularly, the invention relates to a girder assembly for joining two, laterally offset, longitudinal girder members of a light-metal vehicle body.
2. Description of the Prior Art
From German Patent Document DE-OS 37 07 554 there is disclosed a hollow section vehicle frame rail assembly comprising a first, upper U-shaped channel member, a second, lower U-shaped channel member, and an intermediate corrugated sheet in the form of a ribbed plate which is secured between the upper and lower channel members. The ridges and furrows of the corrugated plate preferably include flat regions which are joined together by webbed panel portions that span the distance between opposing walls of the rail assembly. The webbed panel portions act as stiffeners and are oriented at an acute angle with respect to the transverse axis perpendicular to the longitudinal axis of the rail assembly.
This document teaches to maximize the resistance to bending moments along the longitudinal axis of the rail by designing the webbed panel portions with a trapezoidal shape. The stiffness of the rail assembly is thus affected by the acute angled webs which span the distance and take up substantially the entire free space region between the upper and lower U-shaped channel members.
The arrangement as described permits a rail assembly of this kind to be designed with a more favorable overall weight for the case of normal, elastic loading. However, in the case where a vehicle undergoes a heavy impact, a completely different set of stresses occur in the form of plastic deformations, whereby the cross section of the rail assembly of this kind is materially affected. If the cross section collapses because of a plastic deformation of the rail assembly, its stiffness is greatly reduced. The stiffening webs, being set at an acute angle with respect to the transverse axis, provide inadequate stiffness to prevent a sudden collapse of the cross section when plastic deformation occurs. Further, such trapezoidal-shaped webbed panel portions of the ribbed plate are expensive to produce for the case of a sheet metal assembly and further are difficult and/or costly to form as a light-metal castings for use in a light-metal vehicle bodywork. In the case of a light-metal vehicle bodywork in particular, the webbed panels or stiffening walls of this kind would require the additional work steps of insertion within and subsequent fastening to the interior walls of the hollow cross section which necessarily requires further expense.
German Patent Document No. DE 38 39 885 and corresponding European Patent Document No. EP 370 342 A disclose another known vehicle frame girder assembly in the form of a sheet metal longitudinal girder member which is open along one side. The interior hollow portion of this longitudinal member is fitted with an injection molded plastic insert consisting of a row of V-shaped portions. The individual V-shaped plastic portions of the insert function as stiffeners for increasing the flexural and torsional stiffness of the open girder member as compared with an open shell-shaped longitudinal girder member without such an inserted stiffener element. However, in view of the V-shaped arrangement, the stiffeners are of little assistance for maintaining the open cross section of the girder assembly in the event of an impact wherein the girder assembly is likely to experience plastic deformation.
European Patent Document No. EP 0 055 398 A1 discloses another known girder assembly in the form of a sheet metal structure, wherein two sheet metal parts, each having angled ends, are joined together in overlapping arrangement such that they form a closed hollow section. In accordance with the known prior art technique, triangular reinforcement sheets, each provided with welding flanges, are spot welded at spaced intervals along one surface of one of the two sheet metal parts prior to assembly and are further positioned to form inside corner braces for the closed hollow section. This patent document recognizes the tendency of these reinforcement triangles to tear out quickly under transverse load conditions since they are only secured along one side to the bottom wall of the hollow section. Accordingly, this patent document proposes an improved triangular brace arrangement wherein two triangular portions are linked together by a transverse brace member. The brace arrangement has a first surface fixedly attached to the first sheet metal part and a second, abutting surface for supporting by abutment the second sheet metal part so as to prevent the angle of the corner from decreasing below a predetermined value.
From German Patent Document No. DE-AS 22 54 299 a similar stiffener arrangement is known for increasing the bending strength of a closed hollow section of a longitudinal vehicle frame member formed from two sheet metal components. The stiffener arrangement includes bowl-shaped projections which are secured, open-side down, in opposition to the direction of bending such that the bowl-shaped projections abut each other. However, such stiffener arrangements having hollow sections and undercuts are difficult to manufacture as light-metal castings and accordingly are not practical for use in extruded light-metal, hollow section frame members.
All of the above described girder assemblies are designed for use in self-supporting vehicle bodies constructed from sheet metal components. Accordingly, use is made of steel sheet shaped by a conventional deep-drawing process. Hollow section girder assemblies are made from at least two deep-drawn steel sheets which are welded together. The steel sheets used to construct such self-supporting vehicle bodyworks are typically shaped in a deep drawing process. While the dies and pressing tools used for shaping the steel sheets are relatively expensive, they provide a cost-favorable solution for mass production since they permit large production runs. However, in view of the high investment costs for tooling, the aforesaid process is very cost-intensive for smaller production runs.
A more cost-favorable solution for small production runs is known, for example, from European Patent Document EP 0 146 716. This document discloses a vehicle body for a passenger car comprising a bearing structure assembled from a plurality of hollow section frame members or girders which are joined together by nodes or junction elements. The hollow section frame members are formed as extruded light-metal sections and the junction elements are formed as light-metal castings. The light-metal castings may consist of built-up shell portions which, when brought together, form an enclosure for receiving adjoining ends of two or more converging frame members. In addition to being a more cost-favorable solution for small production runs, a light-metal bodywork of the type disclosed in EP 0 146 716 weighs less and is more resistant to corrosion than a sheet metal bodywork.
In the case of such a light-metal bodywork, the longitudinal bearer or girder assembly of the vehicle forecarriage is assembled from bearer members or girders of different profiles. The bearer members are joined by inserting an end of a first bearer member within a receiving end of a second bearer member or by placing them against one another in positive engagement after which they are then fixed in place by welding. A longitudinal bearer member joint is subject to very high loads, particularly during frontal impact. Accordingly, this joint is preferably designed with sufficiently large dimensions to ensure adequate rigidity and strength.
A particularly heavily loaded node element in the bottom portion of a light-metal body of a passenger motor car is the connecting element between the longitudinal bearer or girder member located in the vehicle forecarriage portion and a bottom side sill member. The side sill member is offset laterally outwards with respect to the longitudinal girder because of the front wheel housing. This node element must be Z-shaped in order to bridge the offset. The result of such a Z-shaped configuration is discontinuous or non-linear force flow between the longitudinal girder in the forecarriage and the laterally offset sill member. This is particularly problematic in the event of a frontal impact, since the two force flows diverted in the curves of the Z-shaped path increase the risk of plastic bending in the joint. Under heavy loads, the girder cross section of the Z-shaped node element has a tendency to collapse rapidly thereby greatly reducing the stiffness and supporting function of the vehicle framework.
For the case of light-metal fabrication in general, it is known to provide reinforcement ribs on castings. The ribs are typically situated to lie substantially in the direction of the force flow such that the casting component is stiffened and the stress peaks are reduced thereby.
Until recently, light-metal vehicle bodies have not been manufactured on a large-scale basis. However, in view of recent interest in developing lighter and stronger vehicle bodies, there is a definite need in the art for light-metal node elements of Z-shaped configuration as described above which exhibit improved resistance to plastic deformation and which may be manufactured in a low cost manner.